Obligate heterozygotes for homocystinuria due to cystathionine synthase deficiency have been shown to have elevated plasma concentration of homocysteine-cysteine mixed disulfide after a standardized methionine load. A case-control study reported that after methionine loads 7 of 25 young patients with angiographic evidence of major coronary disease, but only 1 of 22 controls, had homocysteine-cysteine disulfide concentrations as high as those found in obligate heterozygous after similar loading. Together with the well-known tendency of homozygotes for cystathionine synthase deficiency to have thrombi and emboli, these findings suggest that heterozygotes for this condition may be at significantly increased risk for coronary artery disease. A questionnaire survey to evaluate this possibility has been initiated and is now in the pilot-testing stage. BIBLIOGRAPHIC REFERENCE: Uhlemann, E. R., TenPas, J. H., Lucky, A. W., Schulman, J. D., Mudd, S. H. and Shulman, N. R.: Platelet survival and morphology in homocystinuria due to cystathionine synthase deficiency. New Engl. J. Med. 295: 1283-1286, 1976.